Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, facsimile machines, and other types of imaging apparatus. Generally, an inkjet image is formed by selectively ejecting ink drops onto an image substrate from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly. For example, the image substrate is moved relative to the printhead assembly and the inkjets are controlled to eject ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals. The inkjets eject ink in response to receiving the firing signals. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The composition of the ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or other ink compositions, which are stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form and delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink, which is supplied to a printhead.
During the operational life of an inkjet printer, inkjets in one or more of the printheads may become unable to eject ink in response to receiving a firing signal. The inoperative condition of the inkjet may temporarily persist such that the inkjet becomes operational after one or more image printing cycles. In other cases, the inkjet may remain unable to eject ink until a maintenance cycle is performed. Execution of a maintenance cycle, however, requires the printer to be taken out of its image generating mode. Thus, maintenance cycles affect the throughput rate of a printer and are preferably performed during printer downtime.
Numerous types of compensation methods have been developed that enable a printer to print images of an acceptable image quality even though one or more inkjets of a printhead are unable to eject ink. In one compensation method, which is sometimes referred to as a corrective operation, an image rendering process is used to help control the generation of firing signals for operable inkjets. The rendering process modifies input image data, which is sometimes referred to as raw image data, to generate output image data. The output image data are used by the printhead controller to generate firing signals. The compensation method uses information identifying the inoperative inkjets to transition the output image data that corresponds to inoperative inkjets to output image data that corresponds to operable inkjet(s). For example, one compensation method may increase the amount of ink to be ejected by nearby operable inkjets to replace the amount of ink that should be ejected by the inoperative inkjet. The printhead controller generates firing signals for the inkjets with reference to the adjusted output image data so the operable nearby inkjets eject an amount of ink in the neighborhood of the inoperative inkjet to help mask the absence of ink not ejected by the inoperable inkjet. Various image types, ink colors, or other printing parameters affect the effectiveness of a compensation method to mask the effects of inoperative inkjets. Consequently, a continuing need remains in the art to develop methods and systems that more robustly compensate for inoperative inkjets in inkjet printers.